Heat sensitive copy sheet



Nov. 16, 1965 HEAT J. L. REITTER SENSITIVE COPY SHEET led NOV. 23, 1962 INVENTOR.

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United States Patent 3,213,166 l HEAT SENSl'I'lVE COPY SHEET John L. Reiner, St. Paul, Minn., assigner to Minnesota Mining & Manufacturing Company, St. Paul, Minima corporation of Delaware n Filed Nov. 23, 1962, Ser. No. 239,807 14 Claims. (Cl. 96-67) This invention relates to the copying of graphic originals, including those printed in various colors as well as black and white, by processes whereby the visible copy is produced through the application of heat. The invention has particular reference to improved copy-sheet materials with which copies of superior image density and contrast are made possible.

Methods have recently been devised for producing copies of printed or other graphic originalsby first preferentially desensitizing a sensitive intermediate film at reflective background areas of the original through reflex` light exposure, and'then` heating the film in contact with;

asuitable receptor or'image sheet which is thereby visibly changed at areas corresponding to the'still sensitive image areas of the intermediate film. Typically, the film may contain a photosensitive aromatic hydroxy reducing agent, the image sheet then containing a water-insolublev silver 3,2'l8,166 Patented Nov. 16, 1965 ICC 2 z image sheet. The visible change occurring on uniformly heating the exposed intermediate film, with its remaining small amounts of photosensitive reducing agent at image areas, in contact with the image sheet containing the small amount of uniformly distributed non-photosensitive reducing agent, is of much greater intensity than could be ing comprising silver soap and a small amount of a light stable reducing agent. FIGURE 2 represents another embodiment in which both of -the reducing-agent cornponents are incorporated with the silver coating.

In the'association indicated in FIGURE 1, the two soap in a single (sheets form/'a heat-sensitive copy sheet product, stable under room-.temperature storage `but reactive to provide dense black image areas on the silver-coated sheet when locally heated, for example by momentary contact with soap. The reducing agent is reactive with the silver soap at moderately elevated temperatures with liberation of free silver, accompanied by a visible change. Exposure to actinic radiation desensitizes the reducing agent, i.e. renders it nonreactive with the silver ion, and thus prevents occurrence of the image forming reaction at the I exposed areas when the two sheets are subsequentlyl heated in contacting face-to-face relationship.

The amount of reducing agent present in the interheated metal type or a heated metal test bar, or by transfer of a heat pattern from a differentially radiation-absorptive original in heat-conductive contact with the comn posite and subjected briefly to intense irradiation in the therefore limited. Accordingly it has beenv found dimcult to obtain image areas having 'sufciently high optical density to provide fully satisfactory `contrast in the final copy. Toners and other such adivants are useful in pro viding darker image areas, materials such as phthalazinone being particularly effective, but such materials havel not completely solved the problem of obtaining fully defined, dense, high contrast images in copies prepared by the methods hereinabove outlined.

As previously notcd,.the reducing agents employed in the intermediate films are photosensitive, being rendered incapable of causing reduction of the silver ion when sut'- ciently 'exposed to actinic radiation. Other known reducing agents for silver ions-do not possess this property of photosen'sitivity, or are so slightly photosensitive as to be essentially, permanently stable under long continued exposure u'der ordi-nary storage and use conditions.v The incorporation of such materials into the silver soap coating of the image sheet causes pronounced darkening of the -sheet when heated. Excellent heat-sensitive copy sheets havey been made containing these light stable reducing agents inlintimate juxtaposition Withy the particles of v tion in the image or receptor sheet coating of certain essentially light-stable reducing agents for silver ion, and in significantly lesser amounts than normally required to produce desired image density on momentary heating-is effective-in providing fully defined, high density images while avoiding any darkening of background areas of the film, Mylar polyester film being thermographic copying procedure. y Paper is a preferrred substrate for the image sheet. Itmay be of any desired weight. The reactive coating may also be applied to other heat-resisting substrates, e.g. wood, glass, and fabric. The transparent carrier of the intermediate sheet will ordinarily be a thin plastic a preferred example. Thin unfilled paper is also useful.

The reactive components will ordinarily be retained in place on the carrier or substrate by means of minimal amounts of film-forming binder materials. Suitable subcoats or prime coats may be used to insure proper bonding of the active coating, particularly on the transparent films. The reactants may alternatively be introduced into fibrous paper-like webs during the formation thereof -and in the substantial absence of any nlm-forming binder components. n Silver soaps of long-chain fatty acids, e.g. silver behenate and silver stearate, are colorless, visibly stable `toward light, insoluble in many volatile liquid vehicles,

and moisture-resistant; and these materials either alone or blended with additional fatty acid are preferred for use in the invention, although many other readily reducible normally solid organic acid salts of noble metals are also useful.

Photosensitive reducing agents which have been found effective include l-hydroxy-rl-methoxy naphthalene, 1- hydroxy-2-methyl-4-methoxy naphthalene, l-hydroxy-4- ethoxy naphthalene, 4,4'dimethoxyl,1dihydroxy2,2' binaphthyl, l,l'-dihydroxy-LZ'-binaphthyl, 1,4-dihydroxy napthtalene, 1,3-dihydroxy naphthalene, 1-hydroxy-4- amino naphthalene, the bis-dihydroxynaphthalene-dimethylmethane condensation product of 1,5-dihydroxy naphthalene and acetone, 1-hydroxy-2-phenyl-4-methoxy naphthalene, the condensation product of two mols of 1,5-dihydroxynaphthalene and one mol of adipoyl chloride.

. More generally, the photosensitive reducing agent may be an alpha-naphthol having attached directly to the hydroxy. substituted aromatic ring a preponderance of electron donor groups which may be alkyl, aryl, alkoxy, aryloxy, hydroxy or amino. Readily volatile photosensitive reducing agents are employed in the intermediate sheet of the two-sheet composite; materials lof lesser volatility are Ti useful in unitary copy-sheets in which all of the reactants are contained in a single layer.

A particularly effective light stable silver ion reducing agent which has given excellent results in the practice of the invention is 2,6-ditertiarybutyl-p-cresol, also known as Ionol or CAO-3. Combined in small proportions with the silver soap and phthalazinone toner in the image sheet coating, e.g. within the approximate limits of 0.01 to 0.4 mol per mol of silver soap, this material reduces the silver ionand produces a visible change when the sheet is heated for a somewhat longer time and at somewhat higher temperature than is normally the case in thermographic copying processes. However the sheet fails to provide the required density of image when so heated, and remains virtually unchanged when heated for a time and at a temperature such as are ordinarily encountered in thermographic copying processes. At considerably higher concentrations of the Ionol, for exam ple at equal molar concentrationswith the silver soap, an excellent light stable heat-sensitive copysheet is produced. The transfer of extremely small amounts of a photosensitive silver ion reducing agent such as 1hydroxy 4-rnethoxy naphthalene to the Ionol-deiicient coated sheet followed by brief heating at thermographic temperatures, i.e. within the approximate range of 90 to 150 C., results in the formation of visible images of high density. The amount of 1-hydroxy-4-methoxy naphthalene required is far less than the additional amount of Ionol which must be employed to provide comparable image density, and the time andternperature requirements are also reduced.

A number of other nonphotosensitive or light stable reducing agents for silver ion have also been found useful in the practice of the invention. It has been found impossible adequately to define these materials in terms of common structure or formula. However it has been found that those active-hydrogen aromatic organic reducing agents are useful which are characterized by requiring an induction period of at least about six seconds at 100 C. before reactingv rapidly with silver beben-ate and phthalazinone to a dense blackmass.

The test is run as follows: a blend of 0.25 g. of silver behenate, 0.25 g. of the reducing agent, 0.05 g. of phthalazinone, and 3 ml. of methylethyl ketone is prepared by thorough mixing, yand a single drop of the mixture is placed on a small thin cover glass and allowed to dry. The glass is placed on the iiat surface of a metal plate heated to 100 C. and the time required to cause rapid darkening of the dry ilm is noted. It is assumed that the sample reaches the plate temperature within the first second, and accordingly no correction is applied for this period.

A convenient means for obtaining the desired thorough mixing is an ordinary electrically driven high speed dental mixer. A few small glass beads may be added to assist in the mixing if desired. In any event the reactants must be thoroughly mixed so that the color change occurs uniformly throughout the entire `lm on the glass slide.

With some materials a gradual darkening is observed over an extended heating period. As an example, the test with p,pmethy1enebis(N,Ndimethyl aniline) requires some 70 seconds to develop full color change. With these materials the test mixture does not undergo the required rapid darkening, in which the full color change takes place within not more than one or two seconds, and usually even more rapidly.

When tested as just described, many of the reducing agents ordinarily suggested for use with silver soap in thermographic heat-sensitive copy sheet formulations cause complete blackening of the test sample within the rst second or at most within two to four seconds; and these materials have been found ineffective for the purposes of the invention. Thus, methyl gallate requires one to two seconds; catechol and pyrogallol require less than one second; protocatechuic acid and spiroindane require four seconds. image sheets coated with mixtures of these compounds and silver soap show pronounced darkening at background areas, or almost no improvement in image area density, when heated in contact with a reflex-exposed intermediate lilm as hereinbefore described.

On the contrary, Ionol, requiring six to eight seconds for the color change in the described test, provides intense blacliimage and clean white background areas when similarly employed. Other useful light stable reducing agents having analogous image amplifying properties, and the time required for them to produce rapid blackening in the test described, include: 2,6-ditertiarybutyl-li-methoxymethyl phenol, 8-9 seconds; 2,6-ditertiarybutyl-li-etbyl penol, 13-14 seconds; 2,6-ditertiarybutyl-4-octyl phenol, 17-20 seconds; p-cresol, 10 seconds; 2,4,6-tritertiaryamyl phenol, 25 seconds; and 4,4'methylenebis(2,6-ditertiarybutylphenol), l2 seconds.

The use of a photosensitive volatilizable reducing agent in the intermediate sheet permits the 'preparation of reproductions of graphie originals capable of providing differential light images and is ordinarily preferred. Other reducing agents may be substituted where heat images are to be employed, as in direct thermographic copying or in printing with heated metal typeor stylus. As an example, the coating on the intermediate film may contain pyrogallol or catechol in place of l-hydroxy-4-methoxy naphthalene, and images may be produced on animage sheet containing silver soap and small amounts of Ionol by locally heating the intermediate lm at image areas in contact with the image sheet. It is also possible to combine other classes of reducing agents with Ionol or i the like and withthe silver soap in a one-sheet heat-sensitive copy-paper which may at the same time be photosensitive if desired. For example, a combination of silvery behenate, phthalazinone, Ionol and 1-hydroxy-4-methoxy naphthalene provides a coating which converts rapidly to an intense black on brief heating ybut which after exposure to actinic light does not darken beyond a Weak brownish black even on prolonged heating.

The following examples will serve further to illustrate but not to limit the invention.

Example 1 A mixture of equal mol percent of silver behenate and behenic acid is prepared by reacting together one m01V of silver nitrate and two mols of sodium behenate, made from commercial behenic acid, in acidic aqueous medium.

The Washed and dried precipitate is in the form of a fine powder fusing at about C. and melting to a liquid at about 175 C. A coating mixture is prepared by mixi the coating assuming a dull brownish black appearance when the sheet is heated, for example for 2 minutes at C.

Increasing the amount of 2,6-di-t-buty1-4-methyl-phenol component to 12.6 parts in the above formula produces a sheet which permanently darkens to an intense black when` briey heated, for example for 6 seconds at 100 C. or on momentary contact with a metal test bar at 120 C., and which therefore is effective as a heat-sensitive copy sheet in the thermographic copying of differentially radiationabsorptive graphic originals.

A light-sensitive film product is prepared by applying to one mil (.001 inch) Mylar transparent polyester film a thin uniform coating of a blend of 50 parts by weight of ethyl cellulose, 1.4 parts of 1-hydroxy-4-methoxynaphthalene, and 0.3 part of erythrosin B. The mixture is applied from solution in acetone. The weight of the dried coating is 0.2 gram per square foot.

The film is placed with its coated surface in contact with a printed original having light-absorptive inked image areas on a reliective white paper background and which is then uniformly exposed through the film, i.e. in the reflex position, to intense illumination from a bank of tungsten filament lamps for a time just suliicient to desensitize the coating completely at the background areas. A typical exposure time may be 12-15 seconds. The image areas are also affected but to a significantly lesser extent due to absorption in the image areas of the graphic original of that portion of the actinic radiation not previously absorbed in the sensitive coating.

The exposed film is next placed with its coated surface against the coated sheet surface and the composite is heated, for example between rolls or platens, for 4-5 seconds at 12S-140 C. A copy of the graphic original is produced on the coated sheet. The image areas are a dense black; the background areas remain visibly unchanged.

Heating a similary exposed film in contact with a coated sheet from which the 2,6-di-t-butyl-4-methy1- phenol has been omitted produces a faint and incomplete copy of the original, with brownish image areas` Films coated with much larger amounts of the 1-hydroxy-4- methoxynaphthalene and exposed to complete desensitization at background areas produce darker but still incomplete image areas when heated in contact with such silvercoated sheets, and require excessive exposure for desensitization.

Example 2 Essentially the same results are obtained when the phthalazinone of the foregoing example is replaced with phthalic anhydride as the toner for the silver image. More specifically, the silver soap mixture differs from that of Example 1 in substituting 2.6 parts of phthalic acid for the 5.2 parts of phthalazinone, and by increasing the amount of 2,6-di-tertiary-butyl-Lt-methylpheibl from 2.2 parts to 8 parts. On heating the reflex-exposed lightsensitive film in Contact with the coated image paper, there is developed on the latter a blue-brown image.

Example 3 An image sheet prepared as in Examples 1 and 2 with a silver soap coating on a paper-like carrier may be converted into a unitary copy-sheet by the application of a further coating over the silver soap layer. A mixture of 0.1 part by weight of 4-methoxy-l-naphthol, 5 parts of ethyl cellulose, and 95 parts of methanol is coated at a 3-mil orifice over the silver soap coating of an image sheet prepared as described in Example 1, and the sheet is dried, forming a unitary heat-sensitive copy sheet with which copies of graphic originals may be prepared by thermographic back-print copying procedures. Such copies may be desensitized against further heat-induced visible change in the unprinted areas by exposure to ultraviolet radiation. Alternatively, the sheet may first be exposed to an ultraviolet radiation image. as by exposure through a negative transparency or a stencil, and the unexposed areas then developed by heating to produce a visible pattern.

1. A heat-sensitive copy sheet product capable o'f providing high contrast dense black images on a white background when briefly heated at image areas to conversion temperature within the approximate range of 90-150" C., said product containing, uniformly applied over a paperlike carrier and in position for heat-induced inter-reaction, reactants comprising a silver soap of an organic acid, a toner for the silver image, a di-tertiaryalkyl substituted phenol first reducing agent for the silver ion in an amount of at least about 0.01 mol per mol of silver soap and in- (i vsufiicient to form with said silver soap a dense image when briefly heated therewith, and a photosensitve different active-hydrogen aromatic organic second reducing agent for said silver ion in an amount no greater than that of said first reducing agent, said first reducing agent being further characterized as forming with an equal weight of silver behenate and one-fifth said weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds preheat at 100 C. before rapidly darkening at that temperature, and said second reducing agent on exposure to actinic radiation being rendered incapable of reducing silver ion on briefly heating with silver soap at 150 C.

2. A heat-sensitive copy sheet product capable of providing high contrast dense black images on a white background when briefly heated at image areas to conversion temperature Within the approximate range of -150 C., said product containing, uniformly applied over a paperlike carrier and in position for heat-induced inter-reaction, reactants comprising a silver soap of an Iorganic acid, phthalazinone toner for the silver image, a di-tertiaryalkyl substituted phenol first reducing agent for the silver ion in an amount within the range of approximately 0.01 to 0.4 mol per mol of silver soap and insufficient to form with said silver soap a dense image When briefiy heated therewith, and a photosensitive substituted alpha-napthol second reducing agent for said silver ion in an amount no greater than that of said first reducing agent, said first reducing agent being further characterized as forming with an equal weight of silver behenate and one-fifth said weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds preheat at C. before rapidly darkening at that temperature, and said second reducing agent on exposure to actinic radiation being rendered incapable of reducing silver ion on briey heating with silver soap at C.

3. In combination for preparing a visible image, a two-sheet composite of an image sheet and an intermediate sheet in face-to-face contact; said image sheet including a layer consisting essentially of a silver soap of an organic acid, a toner for the silver image, and a di-tertiaryalkyl substituted first reducing agent in an amount of at least about 0.01 mol per mol of said silver soap and insufficient to form with said silver soap a dense image on briey heating said image sheet at 150 C. and characterized as forming with an equal weight of silver behenate and onefifth said weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds preheat at 100 C. before rapidly darkening at that temperature; said intermediate sheet having image and background areas, said image areas including a volatilizable activehydrogen aromatic organic second reducing agent for the silver ion.

4. A two-sheet heat-sensitive copy sheet composite capable of providing high contrast dense black images on a White background when briey heated in face-to-face contact at image areas to conversion temperature Within the approximate range of 90-150 C. and comprising: an image sheet including a layer comprising a silver soap of an organic acid, a toner for the silver image, and a ditertiaryalkyl substituted phenol first reducing agent for the silver ion in an amount of at least about 0.01 mol per mol of silver soap and insufiicient to form with said silver soap a dense image when said sheet is briefly heated, said first reducing agent being further characterized as forming with an equal weight of silver behenate and onefifth said weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds preheat at 100 C. before rapidly darkening at that temperature; and an intermediate sheet including a photosensitve volatilizable active-hydrogen aromatic organic second reducing agent for said silver ion in a concentration no greater than that of said first reducing agent and which on exposure to actinic radiation is rendered incapable of reducing silver ion on briey heating with silver soap at 150 C.

5. A two-sheet heat-sensitive copy sheet composite capable of providing high contrast dense black images on a white background when briefly heated in face-tofacc contact at image areas to conversion temperature within the approximate .range of 90-150 C. and comprising; an image sheet including a layer comprising a silver soap of an organic acid, phthalazinone toner for the silver image, and a di-tertiaryalkyl substituted phenol tirst reducing agent for the silver ion in an amount within the approximate rangcof 0.01 to 0.4 mol per mol of silver soap and insuliicient to lor-m ywith said silver soap a dense image when said sheet is bricliy heated, said iirst reducing agent being further characterized as forming with an equal weight of silver behenate and one-fifth said weight ot` phthalazinone a uniformly blended trace deposit requiring at least six seconds prehcat at 100 C. before rapidly darkening at that temperature; and an intermediate sheet including a photosensitive volatilizable substituted alpha-naphthol second reducing agent for said silver ion in a concentration no greater than that of said rst reducing agent and which on exposure to actiaic radiation is rendered incapable of reducing silver ion on briefly heating with silver soap at 150 C.

6. A two-sheet composite as deiined in claim 4 in which the intermediate sheet is transparent.

7. A two-sheet composite as defined in claim 5 in which the intermediate sheet is transparent.

8. A two-sheet composite as defined in claim 6 in which the transparent intermediate sheet includes a transparent carrier film having a coating comprising the photosensitive second reducing agent and a photoreducible dye.

Si. A two-sheet composite as defined in claim 7 in which the transparent intermediate sheet includes a transparent carrier film having a coating comprising the photoscasitive second reducing agent and a photoreducible dye.

10. Animage sheet for forming dense black images on brief exposure, at a conversion temperature within the approximate range of 90 to 150 C., to trace amounts of a volatilizable substituted alpha-naphthol second silver ion reducing agent, said sheet including a layer requiring extended exposure to a temperature in excess of 150 C. to produce a visible change and consistingessentially of a silver soap of an organic acid, a toner for the silver image, and a di-tertiaryalkyl substituted phenol iirst reducing agent in an amount of at least about 0.01 mol per mol of said silver soap and insulicient to form with said silver soap a dense image on briefly heating said image sheet at 150 C., said first reducing agent being further characterized as forming with an equal weight of silver behenate and one-fifth said Weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds preheat at 100 C. before rapidly darkening at that temperature.

lll. An image sheet for tor-ming dense black images on brief exposure, at a conversion temperature within the approximate range of 90 to 150 C., to trace amounts ot a volatilizabie substituted alpha-naphthol second silver ion reducing agent, said sheet including a layer requiring extended exposure to a temperature in excess of 150 C. to produce a visible change and consisting essentially of a silver soap of an organic acid, phthalazinone toner for the silver image, and a di-tertiarya liyl substituted phenol rst reducing agent in an amount within the approximate range of 0.01 to 0.4 mol per mol of said silver soap and insuicient to form with said silver soap a dense image on briefly heating said image sheet at 150 C., said lirst reducing agent being further characterized as forming with an equal weight of silver behcnate and one-fifth said? weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds prcheat at 100 C. bcl lore rapidly darkening at that temperature.

l2. An image sheet comprising a paper-like backing and a coating thereon comprising a silver soap, phthalazinonc. and 2,6-ditertiarybutyl-p-ciesol, the amount of 2,6- ditertiarybutyl-p-cresol being within the approxiate range of 0.0i to 0.4 -mol per mol of silver soap and insuiiicient to form with said soap a dense image on briefly heating said image sheet at 150 C.

13. A heat-sensitive copy sheet product capable of providing high contrast dense black images on a white background when hrictly heated at image areas to conversion temperature within the approximate range of -150 C., said product containing, in position for image-forming heat-induced interreaction, reactants comprising a silver soap ot an organic acid, a toner for the silver image, a ditertiarybutyl alkyl phenol reducing agent for silver ion in an amount of at least about 0.01 mol per mol of silver soap and insufficient to form with said silver soap a dense image when briefly heated therewith and further characterized as forming with an equal Weight of silver behenate and onc-fth said weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds preheat at C. before rapidly darkening at that temperature, and, in an amount no greater than that of said phenol, a photosensitive alpha-naphthol reducing agent for silver ion, which reducing agent on exposure to actinic radiation is rendered incapable of reducing silver ion in a test procedure involving briefly heating with silver soap at C.

14. A heat-sensitive copy sheet product including an image sheet and a transparent intermediate sheet in faceto-face contact; said image sheet including a surface `layer comprising a silver soap of an organic acid, a toner for the silver image, and a 2,-ditertiaryalkyl-4-alkyl phenol reducing agent for silver ion in an amount of at least about 0.01 mol per mol of silver soap and insulticient to form with said silver soap a dense image on brieliy heating an image area of said image sheet, said phenol being further characterized as forming with an equal Weight of silver behenate and one-litth said Weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds preheat at 100 C. before rapidly darkening at that temperature; and said intermediate sheet including a surface layer comprising a photosensitive alpha-naphthol reducing agent for silverfion, said alpha-naphthol being further characterized as being rendered incapable oi' reducing silver ion, in a test procedure involving brieily heating with silver soap at 150 C., by exposure to actinic radiation.

References Cited by the Esaminer UNITED STATES PATENTS 2,910,377 10/1959 Owen IVI-36.9 3,074,809 1/1963 Owen 117-368 3,094,417 6/1963 Workman 117-369 3,094,619 6/1963 Grant 117-36.8

E WlLLlAhl D. MARTlN, Primary Examiner.

MURRAY KATZ, Examiner. 

1. A HEAT-SENSITIVE COPY SHEET PRODUCT CAPABLE OF PROVIDING HIGH CONTRAST DENSE BLACK IMAGES ON A WHITE BACKGROUND WHEN BRIEFLY HEATED AT IMAGE AREAS TO CONVERSION TEMPERATURE WITHIN THE APPROXIMATE RANGE OF 90-150*C., SAID PRODUCT CONTAINING, UNIFORMLY APPLIED OVER A PAPERLIKE CARRIER AND IN POSITION FOR HEAT-INDUCED INTER-REACTION, REACTANTS COMPRISING A SILVER SOAP OF AN ORGANIC ACID, A TONER FOR THE SILVER IMAGE, A DI-TERTIARYALKYL SUBSTITUTED PHENOL FIRST REDUCING AGENT FOR THE SILVER ION IN AN AMOUNT OF AT LEAST ABOUT 0.1 MOL PER MOL OF SILVER SOAP AND INSUFFICIENT TO FORM WITH SAID SILVER SOAP A DENSE IMAGE WHEN BRIEFLY HEATED THEREWITH, AND A PHOTOSENSITIVE DIFFERENT ACTIVE-HYDROGEN AROMATIC ORGANIC SECOND REDUCING AGENT FOR SAID SILVER ION IN AN AMOUNT NO GREATER THAN THAT OF SAID FIRST REDUCING AGENT, SAID FIRST REDUCING AGENT BEING FURTHER CHARACTERIZED AS FORMING WITH AN EQUAL WEIGHT OF SILVER BEHENATE AND ONE-FIFTH SAID WEIGHT OF PHTHALAZINONE A UNIFORMLY BLENDED TRACE DEPOSIT REQUIRING AT LEAST SIX SECONDS PREHEAT AT 100*C. BEFORE RAPIDLY DARKENING AT THAT TEMPERATURE, AND SAID SECOND REDUCING AGENT ON EXPOSURE TO ACITINIC RADIATION BEING RENDERED INCAPABLE OF REDUCING SILVER ION ON BRIEFLY HEATING WITH SILVER SOAP AT 150*C. 